As optical disks as examples of recording media, a compact disc (CD), a digital versatile disc (DVD), and a Blu-ray disc (BD) (registered trademark) are widely used, for example. These disks show a history of high density recording.
Particularly, in the case of BD-level high density recording, a method using a technique called partial response maximum likelihood (PRML) detection is common as a bit detection method.
As is publicly known, PRML is a technology combining a process called partial response and a technology called maximum likelihood detection. The partial response refers to a process of returning an output longer than one bit to a one-bit input, in other words, a process of determining an output according to a plurality of input bits. In particular, a process of obtaining a reproduction signal as a signal obtained by multiplying successive four-bit information bit inputs by 1, 2, 2, and 1 in order and adding the products, which is often used in an optical disk such as a Blu-ray disc, is expressed as PR (1, 2, 2, 1).
Furthermore, the maximum likelihood detection is a method of defining a distance called path metric between two signal sequences, examining a distance between an actual signal and a signal expected from a presumed bit sequence, and detecting a bit sequence in which the distance becomes shortest. Note that, here, the path metric is a distance defined as a distance obtained by adding a square of an amplitude difference between two signals at the same time over all the times. Furthermore, Viterbi detection is used for searching for a bit sequence that minimizes this distance.
The partial response maximum likelihood detection combining the above process and method is a method of adjusting a signal obtained from bit information of a recording medium with a filter called equalizer to become a process of partial response, examining the path metric between an obtained reproduction signal and the partial response of the presumed bit sequence, and detecting a bit sequence in which the distance becomes shortest.
As a method of evaluating the quality of a reproduction signal of an optical disk, a method using a distribution of a metric difference (also called SAM value) that indicates a margin (path selection margin) of path selection of a Viterbi detector on the basis of the PRML detection principle has already been typical.
For example, Patent Documents 1, 2, 3, and 4 described below and the like disclose a signal quality evaluation method having a favorable correlation with an error rate of the PRML even at the time of high density recording of a conventional optical disk.
In any of the cases, in a PRML class to be actually used, several error patterns with statistically high error occurrence frequencies are extracted, index values are obtained for the respective error patterns, and the index values are integrated to construct an index value.
This is because the distribution of the metric difference (a distribution average value and variance) differs for each error pattern and thus cannot be handled as a single distribution.